In the manufacture of semiconductor die such as diodes or MOSFETs or the like, the die structures are completed to the extent possible in a common wafer and are then singulated (separated) as by sawing in a scribe area (“streets”) between and circumscribing adjacent die.
In many semiconductor device products, such as the Flip Fet™ flip chip MOSFET of U.S. Pat. No. 6,653,740 which is owned by the assignee of the present invention, the die are not packaged and need to be otherwise passivated from ambient.
For that purpose, and while the die are still in the wafer a silicon nitride (Si3 N4) layer about 1.2 microns thick is applied to the wafer surface after the front metal has been applied to the die in the wafer. A PECVD process is conventionally used to deposit the nitride layer. The nitride passivation layer was then subjected to a photolithography step in which a mask was processed and opened only to expose the scribe lines which circumscribe each of the die and the contact regions to which contacts (for contact balls in the case of the device of U.S. Pat. No. 6,653,740) are to be formed on the passivated die.
After masking, the exposed nitride areas are dry-etched and the photoresist was stripped.
To then prepare the surface for solder ball formation, the first step was an electroless Ni/Au plating step over the full wafer surface. The plating adheres only to the exposed areas opened in the passivation layer so that the scribe lines were also plated along with the solder ball electrode pads.
As a result, during dicing or singulation of the die from the wafer, the saw blade became clogged or gummed up by the Ni/Au plate layer.
In an attempt to solve this problem, an oxide (TEOS) film (the surface of the wafer has an overlying TEOS layer beneath the nitride passivation) was left on the streets (after the etch of the nitride). This protected the streets from plating of Ni/Au, but a new problem was created. Thus, the TEOS film was cracking during dicing, with some cracks propagating into the die termination areas.
To solve this problem, the TEOS was removed from the street and a street protection photoresist mask was added to protect the streets from plating. This mask was then stripped with hot sulfuric acid after the plating operation. However, with this process, some of the exposed metals of the wafer were also etched, and the additional photo steps added cost to the process.